This invention relates to the field of electrical power generation, more particularly to the field of power generation using compressed gas storage wherein turbulent gas flow is used to provide more energy efficient storage of compressed gas by reducing the energy gradient inherent to viscous flow.
In the field of electrical power generation, electricity is produced in a variety of ways. Where major demands for electricity exist in a metropolis or other community, large baseload electrical generation facilities are used to generate the electricity. These baseload facilities are often large installations, such as nuclear power plants or coal powered electric plants, costing millions of dollars to construct and being relatively permanent once constructed. Although planning and forecasting go into selecting a site for such a baseload facility, unforeseeable changes in demographics and demand for electricity occur. Such changes can render a baseload facility distant from where the facility's power is needed most.
Furthermore, due to safety concerns and political obstacles, it may not be feasible to locate a baseload facility close to a dense urban area or industrial area needing electricity the most. This is especially true when the baseload facility is a nuclear power plant.
Also, if a city has a power failure, it may have to transmit power in from a neighboring city. Thus, in a power shortage emergency, the electricity transmitted in to alleviate the problem originates from a baseload facility far from the power failure. Significant line losses occur from this long-distance transmission of power.
The power industry has approached this problem by transforming the voltage of electricity generated by the baseload facility to high voltages, and transmitting the high voltage electricity along transmission lines to where the power is needed. In this way, a baseload facility may be located in a suitable location and the power transmitted across the countryside to its ultimate use. High voltages are used in transmission since they result in less wasted energy in the form of line loss than do lower voltage transmissions of the same wattage. However, line losses do occur at the higher voltages, leading to a decay of transmission efficiency over long distances. In order to step up or increase the dropped voltage during transmission, it is often required that the transmission lines be routed to connect with other baseload facilities which will step up the voltage. Such routing may be less than optimal since the step up baseload facility may be located away from the most direct path between the transmitting baseload facility and the end use of the power.
Such limitations in power generation and transmission facilities often become most evident during peak demand periods of the day. These peak demands for electricity typically occur during business hours in business and industrial areas of a community; but the peak demand can shift to outlying residential areas in the evening hours. When electricity demand peaks, the strain on the electrical power system can be great, even leading to blackouts or brownouts. Also, peak demand periods can cause overall system voltage and current drops. These drops can lead to decreased operating efficiency of equipment, such as electric motors and computers designed to operate at a fixed voltage. Other problems from these drops include a need for increased size of protective equipment in the transmission and distribution network, increased transformer KVA ratings, and increased magnetism effects within the transmission conductor.
In addition to shifting peaking demands for electricity, a community may grow, increasing the total demand for electricity. Again, the effects of such demand are greatest during peak demand hours. A community could be faced with the dilemma of choosing between restricting community growth, or constructing additional costly baseload power facilities. The latter would require additional power generation facilities to increase baseload capacity, and additional power transmission facilities to increase transmission load carrying capacity. The present invention affords a community the option of avoiding the capital expense of constructing additional baseload power plants and/or constructing large transmission capacity power lines.
Prior approaches use various means for electrical power generation during peak electricity demand periods known as compressed air energy storage, or CAES systems. One such means is disclosed by U.S. Pat. No. 4,275,310 to Summers and Longardner, showing a peak power generation process in which steam turbines drive air compressors which compress air to be stored in underground geological formations. During peak electricity demand periods, the compressed air is used to drive turbines which turn electric generators. U.S. Pat. No. 4,237,692 to Ahrens and Kartsounes discloses a compressed air energy system for electrical power generation. The compressed gas is stored in one of the "four types of underground reservoirs that are suitable for the storage of compressed air. They are: depleted petroleum fields, aquifers, mined rock cavities, and solution-mined salt cavities." Ahrens, Col. 2, lines 1-4. Other systems are disclosed in U.S. Pat. Nos. 3,597,621, 3,988,897, and 4,443,707.
In the field of compressed gas storage, various arrangements of nesting tanks in parallel arrangement have been used in non-CAES related applications. One such arrangement is illustrated in Hill, U.S. Pat. No. 3,847,173. However, typical gas storage systems lead to energy inefficiencies. When a compressed gas is being pumped into the storage tank, it encounters resident gas in the tank. Resident gas, a relatively still body of gas already in the tank, acts somewhat like a wall, against which newly entering compressed gas is recompressed. Likewise, the resident gas is recompressed. This recompression inside the storage tank causes localized temperature rises in the gas due to the work of recompression done on the gases. Such temperature rises cause a higher temperature differential between the gas and the surrounding environment, thus causing greater heat loss to the surrounding environment. This heat energy loss leads to energy inefficiencies in the overall gas compression and storage system.
The prior system, as seen in Hobson, U.S. Pat. No. 4,150,547, has indirectly addressed this problem by surrounding the gas storage vessel with thermal insulation in order to slow the heat transfer to the surrounding environment.
The present invention improves the efficiency of gas compression and storage systems by reducing heat loss to the surrounding environment. Heat loss is reduced by reducing localized temperature rises in the gas storage tanks. This is achieved by introducing a turbulent flow of gas through the storage tanks during the time the tanks are being filled. This turbulent flow is achieved by bleeding or circulating a portion of the compressed gas out of the tank while the tank is being filled. The result is a turbulent flow of gas through the tank during filling which mixes gas in the tank. This mixing of gas causes heat of recompression and other heat energy to be more evenly distributed throughout the tank. Although the net heat energy in the tank remains approximately the same due to the circulating, localized temperature rises, or hotspots, are reduced or eliminated. As such, localized regions of heat transfer to the surounding environment are reduced, thus decreasing energy loss from the system. Furthermore, heat exchangers may be used to cool the circulating gas during filling of the tanks.
Under certain conditions, savings in lost energy are greater when the gas is not stored for long time periods. Over long time periods, assuming the environment surrounding the storage tank is cooler than the compressed gas, heat energy will be lost to the environment. This is true even if turbulent flow in the tank evenly distributes the heat energy in the tank. However, evenly distributed heat energy will lead to a lower temperature differential with the environment at localized hot spots. A lower temperature differential will result in a slower rate of heat loss to the environment. Thus, the present invention is especially suitable to take advantage of this slowed rate of heat loss. The present invention has particularly good application in the area of compressed air energy systems used for peak period electrical energy generation where storage periods are typically less than twenty-four hours.
The compressed gas removed from the tank during filling is either circulated to the compressor train or bled for use elsewhere. When the gas is circulated, it is injected back into the compressor train which originally compressed the gas for storage. Typically, such circulated gas is injected into a low or intermediate pressure compressor stage in a multi-stage compressor train. The circulated gas is then further compressed in a higher pressure stage of the multi-staged compressor and then pumped back into the tank.
The present invention may, instead of circulating compressed gas back to the compressor train which originally compressed the gas, use the compressed gas elsewhere in another device requiring compressed gas. When the gas is bled for use elsewhere, the gas is employed in a means other than the original compressor train. Typically, this use is to drive a turbine engine or to be further compressed in a second compressor train distinct from the original compressor train. Furthermore, when such gas is used to drive a turbine engine, such turbine engine may be used to drive the original compressor train.
Turbulent gas flow through the gas storage vessel is enhanced when the storage vessel comprises a plurality of elongated needle tanks connected in series, through which the compressed gas flows along a flow path. The gas is circulated or bled at the end of the flow path in the series of needle bottles.
Another advantage of the present invention is that it provides an arrangement to reduce the dynamic shock on a gas storage system when highly compressed gas is introduced into the system. The shock created by introducing gases at pressures higher than residual pressure in the system at 2000 p.s.i. and greater can stress the joints, valves, and other parts of a compressed gas storage system. The present invention can help to relieve such stress on the system, prolonging system integrity.
Although the series arranged tanks may be used in any variety of applications needing a supply of compressed gas, a preferred use of the present invention is to employ it in compressed air energy storage systems. Such systems can be used to supplement electrical power generation, especially during peak electric demand periods of the day.
The present invention is an advance over the prior art in that it provides for increased power generation to boost a system's peak load capacity without having to increase the baseload capacity of the baseload electrical power generation facility and without having to increase the transmission load capacity of the transmission lines. The present invention also provides means for stepping up voltage to eliminate line loss occurring during power transmission. These and other advantages are accomplished by locating satellite power facilities on an electrical power grid and apart from a baseload facility. By selectively locating the satellite power facility near an area of peak electricity demand and by coordinating operation of the baseload facility and the satellite power facility in synchronization with the cycles of peak and non-peak electricity demand, the present invention can meet increases in peak electricity demand. Energy can be generated in the form of electricity, transmitted to the satellite power stations at non-peak electricity demand periods, such as the middle of the night, stored as potential energy at the satellite power stations in the form of compressed air in large needle tanks independent of geological formations, converted from compressed air back into electrical energy using a turbine engine driving a generator, and then distributed to electricity consumers closer to the satellite power station.
In this way, energy can be transported or transmitted to outlying areas during the night when demand is low and the transmission lines have surplus load capacity. Also, many baseload facilities perform at optimal efficiency when they are operating close to capacity. Since many of the baseload facilities are not operated close to capacity during low demand periods, during such periods the opportunity to enjoy this optimal efficiency is lost. This is especially true of nuclear power plants.
In the present invention, the satellite power facilities increase the demand for electricity during the night hours. Thus, during night operations (non-peak demand periods), since output is increased, greater efficiencies in operating the baseload facility are realized. The result is that the system begins the next peak period with efficiently generated surplus energy. Furthermore, the energy is already distributed across the grid network, ready and located to be utilized.
Another advantage of the present invention is that electrical energy may be dispatched upon demand to offset peak loads that may spike the system, such as gas turbine starting packages, electric electrode furnaces or system outages due to apparatus failure.
Another advantage of the present invention is that it may be used to replace or supplement power normally provided by equipment which is off-line for maintenance, repair or replacement.
Another advantage of the present invention is that the satellite power facilities are much easier to locate in a given area than a baseload facility or a geologically dependent CAES system. Also, the satellite power facilities are virtually pollution free, pose no danger of nuclear meltdown and can occupy much less space than a baseload facility. Thus, it is easier to selectively deploy a satellite power facility near a high electricity demand area to boost peak power during peak demand periods. Also, due to the present invention's independence from geological formations, it is technologically feasible to locate a satellite facility almost anywhere. The benefits of the present invention are best realized when the geographic distance between baseload facility and satellite power facility is greater than about twenty statute miles. However, benefits of the present invention may also be realized using shorter distances.
In addition to locating the satellite power facilities along a grid network of a community, satellite power stations may be located along a series of electrical transmission lines. As described above, surplus electrical energy generated during low demand periods can be used to recharge the compressed air storage tanks at the satellite power facility. During high or peak demand periods, the compressed air is used to generate secondary electrical energy. This energy is used to step up the voltage which is being transmitted along lines from a baseload facility. The primary electrical energy generated by the baseload facility is partially dissipated during transmission due to impedance in the transmission lines and equipment. Thus, the present invention boosts or steps up the dropped voltage and lagging current. This power factor management is especially useful where transmission distances are long, even reaching distances of twenty, fifty or even several hundred miles. By locating the satellite facility along the transmission lines where the current is lagging the voltage, the impedance in the line is reduced by using a synchronous alternator at the satellite facility to reduce or eliminate the lag. This results in the downsizing of apparatus and/or improving upon the efficiency of existing apparatus and lines, such as transformers, protective equipment, protective relays, capacitors, and electric motors. This allows transmission (and also distribution) voltages to be maintained without relying on transmission inter-ties or load shedding techniques. Also, when the current is out of phase with the voltage, this lag causes undue magnetism around the transmission lines. By reducing or eliminating this lag, the present invention reduces the adverse environmental effects caused by the magnetism.
The present invention also provides a convenient means for conversion of alternating current to direct current or vice-versa. By using an alternating current motor to drive the air compressor, and by using a direct current generator, the CAES system of the present invention may be used to convert AC power to DC power. Conversely, the present invention may employ a direct current motor and an alternating current generator to convert DC power to AC power.